Arctic sea ice has melted dramatically this summer, smashing the previous record. The Arctic has warmed dramatically compared with the rest of the planet, and scientists say that's what's driving this loss of ice.

To be sure, ice on the Arctic Ocean always melts in the summer. Historically, about half of it is gone by mid-September. But this year, three-fourths of the ice has melted away, setting a dramatic new benchmark.

"It didn't just touch the record, it really drove right through it," says Ted Scambos at the National Snow and Ice Data Center, at the University of Colorado. He says the previous record was set in 2007. This year, the additional area that melted is the size of Texas. And the melt season may have a few days to go.

Direct satellite measurements of the sea ice started in the late 1970s, but there's indirect evidence that this decline is a dramatic departure from a much longer period of time.

"I would say it's been a few thousand years since we've seen the Arctic this open," Scambos says.

The Arctic ice has been in decline for several decades now. Weather conditions in the Arctic this summer were a bit out of the ordinary, but nothing dramatic.

"We saw a very early onset to the melt season, about 10 days to two weeks earlier than usual," Scambos says. And at the summer solstice, when the sun was at its highest, the skies were mostly clear, allowing more sunlight to heat the ocean and melt the ice.

"And by August, we saw an astonishing rate of decline, mostly because the ice was very thin and melted out after three months of warm weather," Scambos says.

This melting trend is accelerating because the ice in the Arctic is getting thinner as the region warms. A few decades ago, lots of ice in the Arctic was 10 feet thick and would clump up as wind pushed it around the northern coastlines.

"That ice used to survive and stir around in the Arctic for decades and create a very thick mass that could survive a few warm summers," Scambos says. "We don't get that anymore. We get persistently warm summers that have gradually eroded the ice cover until it's very, very thin and not stable."

Scambos isn't the only one startled by this abrupt decline.

"It doesn't take a scientist to look at what's happened to the Arctic sea ice to know that something really huge is happening in the climate system," says Jennifer Francis at Rutgers University.

She says the effects of this change won't be confined to the Arctic. Polar bears and walruses, which depend on floating ice, may feel the change most acutely. But changes in the far north are likely to affect our weather as well. She says Arctic warming changes the way waves of weather flow across the Northern Hemisphere.

"Those waves that storms are associated with are tending to move more slowly. And what this means is that increases the probability of some kinds of extreme weather that are related to weather conditions that hang around a long time."

Think of those slow-moving snow storms that buried the East Coast a couple of winters ago. She expects some extreme weather this coming winter, though there's no telling whether it will hit us or someone on the other side of the globe.

Scientists still make projections for when the Arctic could be completely free of summer ice, and those mostly point to the 2030s or beyond. But Scambos says it's a mistake to pay too much attention to the date when the summertime Arctic will be completely free of ice.

"Nobody's going to care that there's a small patch of say 1 or 2 million square kilometers in the Arctic," he says, "because it will be off to one side."

If you care about sea lanes through the Arctic Ocean — or want to understand the climactic impacts of open water in the Arctic — you don't have to wait to see those effects.

Yet, even in extremely warm years like this one, ice is still a force to contend with. Some of the key navigational passages never did open up this summer. And a chunk of ice 30 miles long is heading toward Shell Oil's drilling site in the Arctic Ocean. That has forced the company to move its rig out of the way, just a day after it started drilling.